A variety of devices are used to determine the location and orientation of concealed underground objects. Determining the location of such concealed objects as underground gas, sewer and water pipes, power cables, and telephone and CATV cables or conduits is a necessary prerequisite to excavation and/or the laying of new lines, pipes or cables. For simplicity, these underground objects are hereinafter referred to as underground "lines."
In some applications, an underground steerable boring tool is utilized to form an underground tunnel through which underground lines are subsequently routed. Using a known boring technique, the path followed by the tool under the ground is determined by the orientation of the tool's cutting face. In order to steer the tool, an operator rotates a drill string that is connected to the tool until the cutting face is positioned to attain the desired steering direction. To help the operator steer the tool, a "roll sensor" that determines the angular orientation of the cutting face with respect to the tool's boring axis is installed in or behind the head of the tool. The roll angle data from the sensor is then relayed to the tool operator. The provision of roll angle data has been accomplished by a number of methods such as hard-wired signal transmissions from the boring tool to the operator, acoustic impulses representing roll angle data transmitted along the drill string, pulsation of drilling fluid, or simply by transferring a mark on the drill string. There are a number of problems with these and other conventional techniques, which are discussed in commonly-assigned, copending application Ser. No. 837,110, filed Feb. 18, 1992, now issued into U.S. Pat. No. 5,174,033, which is a continuation application of U.S. Ser. No. 539,699, filed Jun. 18, 1990, now abandoned, entitled "Angle Sensor For A Steerable Boring Tool", which is expressly incorporated herein by reference. These problems include cost, complexity, reliability, and the effects of vibration. However, as described in more detail below, the present invention solves this problem by encoding roll angle and/or pitch angle information onto the signal radiated from a small transmitter located in the boring tool and detecting the information with the same aboveground receiver used to locate and trace the boring tool.
While utilizing a steerable boring tool, it is important for an operator to trace or keep track of the relative location of the existing underground lines with respect to the boring tool, in order to avoid contacting the existing lines with the tool. In other applications, a trench is excavated and lines are subsequently placed in the open trench. While excavating these trenches, it is equally important for an operator to know the locations of any existing lines in order to avoid damaging them with the excavating equipment.
Special-purpose electromagnetic signal detector systems, which are commonly called "locator systems," have been used for many years to locate concealed underground lines, or more recently, to trace the path of a boring tool. Various types of locator systems exist, but receivers that detect electromagnetic signals radiating from either the underground lines or a small transmitter located within the boring tool are by far the most widely used. Such radiated signals are generally produced in two ways: (1) an alternating current signal from an aboveground transmitting source is induced into an underground conductive line which generates an electromagnetic field around the line, or (2) a signal is radiated from a small transmitter either mounted inside a boring tool or positioned within a non-conductive line.
Generally, two types of signal sources will induce a current in a conductive line which, in turn, will generate an electromagnetic field around the line: active signal sources and passive signal sources. An example of a passive signal source in a locating environment is the signal radiated from a very-low-frequency (VLF) radio broadcast station. When such a signal encounters a portion of a buried conductive line, the signal induces a current in the line, which generates an electromagnetic field around the line. Such a source is called a passive signal source, because it requires no operator intervention to generate the electromagnetic field. The problem with a passive signal source is that the same signal may be induced into many different lines, which complicates the operator's task of distinguishing between the different lines.
Conversely, an active signal source is intentionally utilized by an operator to generate an electromagnetic field directly associated with the object to be traced. For example, an operator may couple a signal having a known frequency to an underground cable, for the purpose of generating a distinct electromagnetic field around the cable. The presence of the distinct electromagnetic field allows the operator to locate the cable and distinguish it from other cables with an aboveground receiver. This ability to distinguish between the cables is a key advantage of an active signal source over a passive signal source. Another example of an active signal source is the small, subterranean transmitter which may be installed in a boring tool or passed through a hollow non-conductive line. A distinct signal radiated from the subterranean transmitter allows the operator to trace either the path of the boring tool or that of the non-conductive line. It is noted that the present invention deals only with the use of active signal sources.
As discussed above, a key advantage of an active signal source is the capability of coupling a distinctive frequency signal onto one conductive line, and distinguishing that particular line from adjacent or nearby lines. Consequently, the conductive line of interest can be traced with less confusion or interference from the adjacent lines. Since the frequency of the coupled signal can be controlled precisely, a narrow bandwidth can be used for greater selectivity in the receiver. Also, the use of a narrow bandwidth improves the signal-to-noise ratio and increases the sensitivity of the receiver. The use of a narrow bandwidth in a locator receiver can be especially important for locating conductive lines in the vicinity of a strong radio transmitter, where the airborne signal can swamp out a subterranean signal unless the airborne signal is filtered out by the receiver's selective, narrow-band circuitry. Another advantage of an active source is that locator calculations of the position and depth of underground conductive lines are not affected by electromagnetic field distortions from multiple signal sources to the same degree as they are with a passive signal source.
The most practical way to couple a signal to an underground conductive line is simply to attach a wire directly from the transmitter to the line. Such a technique is illustrated in U.S. Pat. No. 4,387,340 to Peterman. If this approach is not feasible, it may be possible to attach the transmitter wire to a toroidal clamp, which is placed around the circumference of the line in order to induce a signal current into the line. The signal induced into the line is then radiated from the line and detected with a locator receiver. Alternatively, if the signal cannot be readily coupled directly to the line because, for example, the line is completely buried, the signal can be coupled indirectly to the line by using a coil located in the transmitter and passing an alternating current signal through the coil. The electromagnetic field from the coil in the aboveground transmitter radiates through the earth and induces a current into the buried line. The signal induced into the line is then reradiated from the line and detected with a locator receiver.
As shown in Peterman, an active signal source is commonly used when an operator desires to locate and trace a specific underground line that is near numerous other lines. A distinctive frequency signal is coupled from a locator transmitter to the line to be traced. For example, the transmitter generates a signal at a specific frequency. A locator receiver is manually set to the frequency of the transmitted signal, and the receiver operator can thereby distinguish the particular line which is radiating the transmitted signal from the other, nearby lines.
More specifically, FIG. 1 illustrates a perspective view of a conventional aboveground locator system utilizing an active signal source. Transmitter 10 is positioned on the surface of earth 15 above buried line 20, which is the concealed object to be traced. Transmitter output connector 12 is connected to wire 18, which is in turn connected to conductive line 20. The connection of wire 18 to line 20 may be accomplished by directly attaching wire 18 to line 20, thus providing an electrical connection therebetween, or by connecting a toroidal clamp (not shown) to wire 18 and placing the clamp around line 20 to thereby induce a current. Thus, the output signal of transmitter 10, which is an AC continuous wave (CW) signal, is induced into line 20. Ground stake 16 is placed deep into the earth and connected to transmitter 10 by ground lead 17, in order to provide a ground return path for the signal current induced into line 20. Consequently, the output signal from transmitter 10, which is at a unique frequency, generates an electromagnetic field that radiates from line 20 with a field pattern such as that illustrated in FIG. 2. Alternatively, if line 20 is buried to the extent that it is inaccessible by wire attachment or toroidal clamp, the output signal is coupled to internal coil 11 in transmitter 10, which radiates electromagnetic field 32 corresponding to the CW signal. The radiated signal propagates through earth 15, is induced into line 20, and reradiates from line 20 with a field pattern such as that illustrated in FIG. 2. Receiver 30 is positioned on or above the surface of earth 15 in the general vicinity of line 20 and manually set by an operator to the frequency of the transmitted signal. By sensing and processing the signal radiated from line 20, and using conventional locating techniques, the receiver operator locates the position of line 20 and traces the signal along the line's path.
FIG. 3 illustrates a perspective view of a conventional subterranean type of active signal source locator transmitter which, in this case, is utilized for tracing the progress of a boring tool. It is often necessary to trace the progress of a steerable, underground boring tool in order to guide the tool to its destination and avoid contacting existing lines with the tool. Referring to FIG. 3, transmitter 50 is placed either within or closely behind boring tool 52. Transmitter 50 radiates a signal through earth 15 to aboveground receiver 30. In response to location information provided by the receiver operator, an operator (not shown) of boring tool 52 rotates drill string 54 about its boring axis to control the direction of boring tool 52.
In another conventional application, it is sometimes required to detect and trace the paths of plastic or concrete underground pipes. Since these lines are nonconductive, there is no way to trace them by inducing an alternating current signal in them and detecting the radiated electromagnetic field. Consequently, a small subterranean transmitter is inserted into the plastic or concrete line, and the electromagnetic field radiated from the transmitter is detected by an aboveground receiver and traced by an operator along the path of the line. Referring to FIG. 4, subterranean transmitter 60, an active signal source, includes coil 62 which is wrapped around ferromagnetic rod 64. Coil 62 is energized with a closely controlled signal frequency by oscillator 66. Oscillator 66 is powered by an internal battery (not shown). Subterranean transmitter 60 is attached to rod 68 and pushed down the length of plastic or concrete line 65. Receiver 30 detects and processes the electromagnetic field radiated by transmitter 60. Consequently, a locator operator can trace the position of subterranean transmitter 60 as it is routed through line 65.
Conventional locator systems are relatively inflexible and inefficient from an operational standpoint, because they are constrained to performing only their primary locating functions. Because locating, distinguishing and tracing multiple, concealed underground lines has become increasingly time consuming and costly, a need has existed in the art of locator system design to provide more flexible and less costly locating techniques and equipment. For example, at a relatively large excavation site, it may be necessary to trace different lines utilizing both an aboveground and a subterranean transmitter. Utilizing a conventional active signal source locating system, an operator selects a particular line to be traced, attaches a coupling wire from a transmitter to the line and a lead from the transmitter to a ground stake, and manually selects a transmitter frequency. Either the same or another operator sets up the locator receiver to receive the transmitted signal (i.e., selects the appropriate receiver frequency) and then proceeds to locate and trace the line of interest. Since the conventional locator transmitter is configured to output only one signal frequency at a time, the operator is limited to tracing one line at a time with one transmitter. Consequently, if more than one line is to be traced simultaneously, then a separate transmitter must be attached to each line to be traced. However, the present invention is not so limited by such constraints. For example, in order to reduce the time required to trace a number of separate lines, without incurring the cost of utilizing a multiple number of separate transmitters, the present invention utilizes one locator transmitter capable of coupling a separate identification signal to each of the lines to be traced. Consequently, each line radiates a unique signal that can be identified by an operator utilizing a suitably adapted receiver.
Additionally, it is sometimes necessary for an operator to change the type of transmitter to be used for a particular locator application. If, for example, an operator utilizes a conventional aboveground transmitter coupled to an underground line, the receiver is configured to detect and process the signal unique to that type of transmitter. If the operator is then required to utilize a conventional transmitter located in a steerable boring tool, in order to bore an underground tunnel at the same site, the receiver may have to be configured differently (retuned), or perhaps a different receiver may have to be employed, to detect the transmitted signals from the subterranean transmitter and trace the progress of the tool during the boring process. The present invention is not limited to such a time-consuming process but instead provides a locator system having a transmitter capable of transmitting its own product identification signal to a receiver which, in turn, is capable of automatically reconfiguring itself to operate compatibly with the type of transmitter in use.
Also, from an operational efficiency standpoint, it would be desirable to provide a transmitter located in a steerable boring tool, such as that described in the example above, which is capable of encoding the angular position information from the boring tool onto the transmitted signal. Moreover, it would be desirable to transmit the angular position information in digital form, in order to minimize the effects of noise and interference on the transmitted signal. Such a digital encoding capability is not presently available with conventional locator systems. However, as described in more detail below, the present invention provides such novel capabilities.
Finally, conventional active locator systems as shown in FIG. 1, provide no direct communications link between the transmitter and receiver. Consequently, for example, if a transmitter operator wishes to direct a receiver operator to change the receiver's frequency to correspond to a new transmitter setting, or to communicate some other type of operational information to the receiver operator, a two-way radio or field telephone is typically used. Alternatively, the receiver operator may have to travel to the location of the transmitter and personally converse with the transmitter operator, which is a time-consuming alternative. Therefore, it would be desirable to provide a locator system that is capable of transmitting a voice signal over the electromagnetic field radiating from an underground line, in order to provide a built-in communications link. Moreover, while locating lines at a site where only one person is employed to operate both the transmitter and receiver, it would be desirable to provide a system that can transmit an audible tone or other type of analog signal to the operator at the receiver, which indicates certain operating characteristics of the portable transmitter such as, for example, battery power level. No such capability is presently available in conventional locator systems. However, the present invention provides a built-in communications link between an aboveground locator transmitter and receiver that includes such an analog signal and/or voice transmission capability.